Burned Out Battery (Alternator) Light Indictor...Major Drain

E28 technical advice asked and given! Troubleshooting, modifications and more.
Post Reply
Preacher's 5
Posts: 186
Joined: Aug 20, 2007 11:50 PM
Location: Decatur, GA
Contact:

Burned Out Battery (Alternator) Light Indictor...Major Drain

Post by Preacher's 5 »

Hey guys, I was wondering how many of you know that if the battery/alternator light bulb blows, that it will cause a negative drain on battery. Yesterday I started my car and let it idle to warm up, but did not realize that when I turned the ignition switch on the battery/alternator light was not on. When I turned the car off, it would not start. First I thought the battery was dead. So I waited about 10 minutes and the cars started right up. Then I thought I needed an alternator. Today, when checked the battery and alternator voltage, it was reading 11.2 volts. I know that it should 14. I called my mechanic, Wayne, and he told me to turn the ignition switch on and see if the battery/alternator light was on, I told him..NO! He told me to pull the cluster out and replace the bulb, then check to voltage, which I did...now it is reading 14.02 volts. WOW! Problem solved! Saved a couple hundred bucks! Car is cranking and running great.
I have owned this car for 35 years and had never heard that before. You learn something new every day. I wonder how many people have replaced their alternator, (as I was about to do); or replaced the battery because of a $2 bulb, (which I have a bunch of). Just curious and passing on some info. I truly thank God for my mechanic, Wayne Lins. Great car brain.
Shawn D.
Beamter
Beamter
Posts: 21290
Joined: Feb 12, 2006 1:00 PM
Location: Alpharetta, GA
Contact:

Re: Burned Out Battery (Alternator) Light Indictor...Major Drain

Post by Shawn D. »

This is fairly well-known, but it's not intuitively obvious. This was also an issue on GM cars that used a similar circuit to excite the alternator.

FWIW, later instrument clusters have a resistor parallel to the bulb so that if the bulb blows, the alternator will still get power from that circuit.
Preacher's 5
Posts: 186
Joined: Aug 20, 2007 11:50 PM
Location: Decatur, GA
Contact:

Re: Burned Out Battery (Alternator) Light Indictor...Major Drain

Post by Preacher's 5 »

Shawn D. wrote: Feb 17, 2021 9:27 PM This is fairly well-known, but it's not intuitively obvious. This was also an issue on GM cars that used a similar circuit to excite the alternator.

FWIW, later instrument clusters have a resistor parallel to the bulb so that if the bulb blows, the alternator will still get power from that circuit.
Thanks Shawn. 35 years and never had the issue until yesterday. Glad to know some-one else is up on that revelation.
BMWCCA2
Posts: 3712
Joined: Feb 12, 2006 1:00 PM
Location: Central Virginia

Re: Burned Out Battery (Alternator) Light Indictor...Major Drain

Post by BMWCCA2 »

It was a fairly common issue on 2002s, too.
demetk
Posts: 8255
Joined: Aug 09, 2007 8:58 AM
Location: CT, USA

Re: Burned Out Battery (Alternator) Light Indictor...Major Drain

Post by demetk »

These cheap voltmeters are useful tools, if and when you remember to look at them :)

Image

Image
indycar89
Posts: 250
Joined: Apr 03, 2012 5:21 PM
Location: Indianapolis, IN

Re: Burned Out Battery (Alternator) Light Indictor...Major Drain

Post by indycar89 »

demetk wrote: Feb 19, 2021 7:12 PM These cheap voltmeters are useful tools, if and when you remember to look at them :)

Image
Nifty meter in the check panel - remember what/where you found it?
demetk
Posts: 8255
Joined: Aug 09, 2007 8:58 AM
Location: CT, USA

Re: Burned Out Battery (Alternator) Light Indictor...Major Drain

Post by demetk »

indycar89 wrote: Feb 22, 2021 6:39 PM Nifty meter in the check panel - remember what/where you found it?
https://www.amazon.com/gp/product/B00YA ... UTF8&psc=1

My check panel came apart easily so installing the meter wasn't difficult.
gadget73
Posts: 639
Joined: Nov 22, 2017 11:30 PM
Location: New Jersey

Re: Burned Out Battery (Alternator) Light Indictor...Major Drain

Post by gadget73 »

Shawn D. wrote: Feb 17, 2021 9:27 PM This is fairly well-known, but it's not intuitively obvious. This was also an issue on GM cars that used a similar circuit to excite the alternator.

FWIW, later instrument clusters have a resistor parallel to the bulb so that if the bulb blows, the alternator will still get power from that circuit.
Fords do this too. Not so much a drain, just it doesn't turn the alternator on. There is a resistor but its on the cluster so it doesn't work if you have that out for whatever reason.

I had a Beetle years ago that wouldn't charge, same thing with the bad bulb.
craigb93
Posts: 715
Joined: Feb 12, 2006 1:00 PM
Location: Cumming, GA

Re: Burned Out Battery (Alternator) Light Indictor...Major Drain

Post by craigb93 »

indycar89 wrote: ↑22 Feb 2021 18:39
Nifty meter in the check panel - remember what/where you found it?

https://www.amazon.com/gp/product/B00YA ... UTF8&psc=1

My check panel came apart easily so installing the meter wasn't difficult.

Where were you able to get the voltage signal from? Inside the Check Panel or ?
demetk
Posts: 8255
Joined: Aug 09, 2007 8:58 AM
Location: CT, USA

Re: Burned Out Battery (Alternator) Light Indictor...Major Drain

Post by demetk »

craigb93 wrote: Feb 24, 2021 9:40 PM
Where were you able to get the voltage signal from? Inside the Check Panel or ?
I may have spliced at the check control connector. I could check if needed.
LeiseyJr
Posts: 1327
Joined: Dec 22, 2013 11:11 PM
Location: Houston,Tx

Re: Burned Out Battery (Alternator) Light Indictor...Major Drain

Post by LeiseyJr »

A spare bulb is always in my glovebox. However when my light did not come on it was due to to a dead alternator.
craigb93
Posts: 715
Joined: Feb 12, 2006 1:00 PM
Location: Cumming, GA

Re: Burned Out Battery (Alternator) Light Indictor...Major Drain

Post by craigb93 »

by demetk » 24 Feb 2021 23:20
craigb93 wrote: ↑24 Feb 2021 21:40

Where were you able to get the voltage signal from? Inside the Check Panel or ?
by demetk » 24 Feb 2021 23:20
I may have spliced at the check control connector. I could check if needed.


Sp, system voltage. Would be kind of messy to wire for alternator output anyway.

Thank you. -Dick
Post Reply